Investigation of Flame Detachment Effect during Early Flame Development in a Swirl Flow Field 2021-01-0482

Lean burn is regarded as one of the most effective ways to improve fuel efficiency for spark ignition engines. However, the excessive air dilution deteriorates combustion stability, limiting the degree of engine operational dilution. The intensified flow field is therefore introduced into the cylinder to mitigate the decline of the burning velocity caused by the leaned-out fuel-air mixture. In a moderate flow field, flame kernels are formed near the hot spark plasma during discharge and stick to the spark gap even after the end of discharge; the flame front then propagates outward and evolves into self-sustained flame. Flame attaching to the spark gap is a common phenomenon in the early combustion stage and has been reported to be beneficial for flame inception in the literature. However, it is also observed that the formed flame kernels sometimes are blown away from the spark gap by the strong flow motion, and the detached flame kernel gradually develops to self-sustained flame without touching the spark gap. The flame detachment phenomenon has been studied in some literature. Nevertheless, what causes flame kernel detaching from the spark gap and how this phenomenon affects the early combustion process are still less understood.

In this paper, an investigation of the flame detachment phenomenon during early combustion is carried out in a constant volume swirl chamber with optical access. The impacts of various parameters on flame detachment are studied, including gas composition, discharge duration, and discharge current amplitude. Electric waveforms during discharge are acquired to characterize the discharge process; shadowgraph images are used to depict the flame detachment phenomenon under each experimental condition.

Results have shown that flame kernel detachment is mostly affected by discharge duration and flow speed, while less impacted by discharge current amplitude and gas composition. With the same amount of discharge energy delivered to the spark gap, a longer flame attachment duration does not necessarily always have a positive impact on combustion.